But the truth about what happened that fateful night of Dec. 23, 1888 — did the painter really cut off his whole ear in a fit of madness, or just sever his earlobe? — divided art historians for decades. That is, until last summer, when amateur historian Bernadette Murphy published a book that included a drawing from the doctor who treated Van Gogh after the incident, showing he sliced off everything except a nub of his left lobe.

Now the book’s companion documentary, “Secrets of the Dead: Van Gogh’s Ear,” premiering Wednesday at 10 p.m. on PBS, is taking a closer look at the mystery, and revealing how it took a British expat living in France to uncover the evidence that finally cracked the case.

Murphy, 58, moved to the French region of Provence from the UK 33 years ago, and lives in a village of 800 people located about an hour’s drive from Arles, where Van Gogh lived and worked for 16 months in 1888-89, creating nearly 200 paintings. She visited the town often with family and friends, and talk always turned to Van Gogh.

“I would be wandering around the garden in Arles where the archives are, and I’d hear some guide saying, ‘He cut off the lobe and not the whole ear,’ and then you’d hear another one saying, ‘He cut off the whole ear,’ and I thought, ‘Why don’t we know?’ It seemed silly to me,” Murphy tells The Post.

“We really knew practically nothing about what happened that night even though the police were called; there should have been medical records.”

Van Gogh, then 35 years old, had been showing erratic behavior that December, so much so that he had driven his housemate, artist Paul Gaugin, out of their burgeoning artists colony after just two months. Then, on Dec. 23, Van Gogh received a letter from his brother Theo in Paris announcing his engagement — spurring worries about losing his sibling’s financial and emotional support, and apparently, driving him to slice into his own ear.

Bernadette Murphy, author of “Van Gogh’s Ear, The True Story”

When Murphy fell sick in late 2008 and was stuck at home, she started to read up on the incident. Intrigued by inconsistencies she noticed in the story — and motivated to achieve something important following the recent death of her sister — she quit her job teaching at a local art school and spent the better part of the next seven years uncovering evidence and writing about her discoveries in the book “Van Gogh’s Ear.”

“Living locally, I realized there were small mistakes in the story, small points that didn’t hold true,” she says. “The only way to look at it was as a detective might — as a cold case — and start all over again. So that’s what I did. I didn’t think it would take me quite so long.”

Murphy was curious from a young age; she grew up outside of London in a working-class family, the youngest of eight children who were all encouraged to read and use the public library. She especially credits her experience doing genealogical research into her Irish family tree for giving her useful tools to look beyond official records.

When she started her Van Gogh research, only one press account of the fateful evening existed — she dug up four more (including one that corroborated his use of a razor) by spending months making phone calls and visiting more than 50 newspaper archives (many of which weren’t digitized). From reading the artist’s letters, she knew that he would sign his name “Vincent” because no one could pronounce his Dutch surname — and therefore some of the newspapers had previously been overlooked because they didn’t include the name “Van Gogh.”

A diagram drawn by Van Gogh’s physician, Dr. Felix Rey, showing how he severed all of his ear except the lobe.Bancroft LIbrary

She found the so-called “smoking gun” of the mystery — a diagram drawn by Van Gogh’s physician, Dr. Felix Rey, showing how he severed all of his ear except the lobe — a year into her research, in January 2010. The clue to the bombshell discovery lay in the research library at the Van Gogh Museum in Amsterdam: a previously unnoticed 1955 letter from a Time magazine editor, replying to a reader who had complained their article made a mistake when mentioning Van Gogh cut off his whole ear. The editor defended the characterization, stating the author Irving Stone, who wrote a 1934 biographical novel about Van Gogh called “Lust for Life,” had a drawing in his possession that showed exactly what the artist did.

“I just kept thinking if it existed in this letter of 1955, then surely it must still exist,” Murphy says.

She tracked Stone’s archives to The Bancroft Library at the University of California, Berkeley, and started emailing its archivist, David Kessler, to search for the document in question. (Since the small drawing was folded in half, she had to pester him to look five times before it was unearthed.) But even after making this seismic discovery, she kept quiet, patiently buttressing her case until unveiling the sketch at the Van Gogh Museum last summer.

“I knew it was worth something, but I knew that nobody knew who I was, and if I was going to prove it, then I had to look at it in every single way, to prove it irrefutably,” she says. “I had to find out where the story had gone wrong because they had always believed something else.”

Measuring the framed Dr. Felix Rey drawing

Murphy studied Van Gogh’s self-portraits from the time for clues, consulting a local emergency-room physician who told her the painted wads of padding on the artist’s bandage indicate more bleeding than would come from just a severed lobe. She reread Van Gogh’s letters, in which he describes having an infection and running a fever — consistent with a traumatic injury in a time of no antibiotics — and that he had left the hospital after two weeks, indicating a complicated recovery, as well as sustained blood loss.

The persistent belief that Van Gogh cut off just the earlobe originated from an eyewitness account from his friend, the artist Paul Signac, who went to visit him shortly after the 1888 incident. In a 1921 letter, he recalls Van Gogh “cutting off the lobe and not the whole ear.”

“I read French, [and] what is never published is [Signac’s] full letter, just that small excerpt,” Murphy says. “His full letter goes on to say when he saw him for the last time, he was ‘dressed as usual, wearing a band ’round his head and a fur cap.’ I went ‘Bingo’ — he never saw the ear uncovered because he had a band around his head.”

When she looked back at other accounts, they also described Van Gogh as always covering his ear — leading her to attribute Signac’s wording as a simple transposition that had come to be taken as gospel.

“Probably, Paul Signac slightly misunderstood him because all that was left was the lobe,” she says. “Once I understood that he never saw it uncovered, then . . . the whole story held up about the blood loss.”

There was still another major question to answer — the identity of the woman to whom Van Gogh delivered his severed ear at the brothel that night, always known simply as “Rachel.” But when Murphy scoured census records from the time and found no prostitutes named “Rachelle,” she noticed one of the newly unearthed press reports named her as “Gaby” — and sure enough, she found Gabrielle listed.

She called a few people in Arles and managed to track down the woman’s descendants, and though they didn’t want to be identified, they told Murphy that Gabrielle had been bitten by a rabid dog and sent to Paris for treatment in January 1888, where Louis Pasteur had just discovered the rabies vaccine. When she returned to Arles, she began working as a maid — not a prostitute — at the brothel to help pay off the medical bills.

Murphy uncovered her medical file, and on re-examining Van Gogh’s letters, she found a passage referring to women with rabies living at the Institut Pasteur — a coincidence that led her to theorize that perhaps the artist and Gabrielle first met in Paris, shortly before he decamped to Arles in February 1888.

But unlike Murphy’s ear discovery, that potential first chapter of this saga remains just a theory.

“Nobody knows why Vincent went to Arles, of all the places in the world; the logical place would have been Marseille [the city that inspired impressionists such as Paul Cezanne]. Maybe they did meet in Paris,” Murphy says. “It’s not something you can prove, it’s just a possibility.”